Ship parts

Howdy, trying to make use of branches, not sure If I got it right.
I have finished with this branch, now the next thing to do is a pull request right?

Removed ship sprites and replaced them with procedural shapes...
I’m pretty sure that `path` is getting leaked. Unlike Cocoa objects, Core Graphics APIs return types which you have to retain/release yourself.

One way to fix that is to rename the methods returning `CGPathRef` to `new<Whatever>`, e.g. `newHull`, and then in the caller, make sure that it’s creating a path, assigning it wherever it needs to go, and then releasing it:

```objc
CGPathRef path = [part newHull];
node.path = path;
CGPathRelease(path);
```

The `new` prefix is an idiom which Objective-C uses to mean “returns something which the caller will have to release.” Objective-C actually enforces that for e.g. Cocoa objects, and for stuff like Core Graphics types it’s good documentation.